The invention relates to a method for the manufacture of the cylindrical mantle of the rolls or cylinders of a paper machine out of a corrosion-proof metal or alloy material, particularly of refined steel, and particularly for the manufacture of the cylindrical mantle to be perforated for a suction roll of a paper machine.
The invention also relates to a roll or cylinder of a paper machine, in particular a suction roll provided with a perforated mantle, comprising a cylindrical mantle and end pieces attached to both of its ends, in connection with which end pieces there are the axle journals of the roll or cylinder.
Rolls of paper machines, inparticular suction rolls, operate in an environment which is highly corrosive. Moreover, suction rolls, in particular press rolls, are subjected to high dynamic loads, because at present the linear loads employed, e.g., in press rolls are of an order of from about 70 kN/m to about 120 kN/m or even higher. For this reason, in the mantles of suction rolls or the equivalent, it is necessary to use extensively alloyed special steels, such as two-phase, i.e. so-called duplex steels, which are expensive and difficult to work when cold. Problems of strength in the case of the mantles of suction rolls are also caused by the fact that their mantles are perforated, one mantle comprising typically about 500,000 suction holes.
The diameters of prior art suction rolls are generally of an order of from about 600 mm to about 1400 mm, and their wall thicknesses are between about 55 mm and about 90 mm, in the case of large paper machines usually between about 70 mm and about 90 mm. The lengths of the suction rolls correspond to the width of the paper machine, being usually within a range of from about 5 m to about 10 m.
The cylindrical mantles of suction rolls or equivalent for paper machines are, in prior art, manufactured by means of the following techniques. The cylindrical mantles are bent by being rolled out of a plate almost to the shape of a full circle or a semicircle, and the longitudinal joint or joints is/are welded together. Also, it is obvious that the mantles to be bent must have either a thin wall (such as an iron wall) or be formed from a readily yielding material (such as bronze). Mantles having a thick wall or formed from hard materials would not be useable in this type of prior art manufacturing process.
Correspondingly, it is known in the prior art to bend cylindrical mantles from a plate to a curved shape by chamfering to a semicircular shape, whereupon the longitudinal joints of the cylinder halves are welded together. Chamfering can, as a rule, be employed up to a mantle wall thickness of about 50 to about 70 mm only. Hereupon the cylindrical mantle made by rolling or chamfering is machined to cylindrical shape.
A particular drawback of the prior art mantle bending techniques, such as that described in U.S. Pat. No. 3,186,063 (Dopp), is that the bending of the flat mantles to a new larger radius of curvature causes tension in the inner surface of the mantle blank and compression in the outer surface of the mantle blank. This effect is an obvious result from the outward bending of the mantle blanks as described in the Dopp reference.
In the prior art, cylindrical mantles for paper machine rolls or cylinders are also manufactured by means of centrifugal casting. In this casting process, the casting mold is made to revolve in a horizontal position, e.g., on rolls, and molten metal is fed into the mold. The metal remains and solidifies on the mold walls by the effect of centrifugal forces.
Since the wall thicknesses of suction rolls and equivalent are quite high (between about 55 mm and about 90 mm), the rolling and chamfering of the plate material requires particularly robust equipment and high forces. In spite of this, for example when rolling is used, the roll mantle must be composed of axial parts of a length of about 2 m to about 3 m. With larger mantle thicknesses, higher than about 70 mm, rolling is not possible except by means of particularly robust equipment or by using very short mantle portions which are hot. Thus, with higher mantle thicknesses, it has been necessary to use chamfering, by which means it is, however, difficult to bring the mantle to precisely circular shape, which results in the drawback that large quantities of material must be machined off the mantle. This increases the time taken by the machining and the loss of material.
In a mantle manufactured by rolling or chamfering, it has been necessary to use transverse welding joints, which has resulted in the following drawbacks. In practice, it has been noticed that the major part of the suction rolls are broken down by breaking off so that mostly the breaking point is exactly at the location of a transverse weld when the roll has been manufactured by welding. The points of starting and ending of a transverse weld are particularly risky problem points. For this reason, one of the main objectives of the present invention is to provide a process for the manufacture of a roll mantle and a roll or cylinder manufactured by means of the method wherein there are no transverse joints at all.
It is a drawback in the use of centrifugal casting methods that about one half of the wall thickness must be machined before a "sound" roll mantle is obtained. It is a further drawback that casting flaws tend to remain in the roll mantle, and these flaws constitute starting points for fractures. A considerable drawback is the above proportion of material lost on machining because the extensively alloyed steels used in roll mantles are very expensive.
It is another particular drawback of a centrifugal casting process that there is an inherent limit on the type of material that can be used, i.e., it must be a material which is suitable for centrifugal casting. Since rolls or cylinders used in paper manufacturing methods are subjected to high dynamic loads, the mantle of the rolls or cylinders must have a relatively thick walls which are difficult to provide using a centrifugal casting technique.